Method of coating an article of manufacturing having a substrate formed of a nickel or cobalt-based superalloy

ABSTRACT

A nickel-based or cobalt-based superalloy substrate is covered with a protective system which is resistant to thermal, corrosive and erosive attack. An anchoring layer is disposed on the substrate. The anchoring layer is formed as an oxide compound, in particular alumina, doped with nitrogen. A ceramic coating is disposed on the anchoring layer. The modification of the anchoring layer prevents the transmission of diffusing elements through the anchoring layer to the thermal barrier layer.

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional application of U.S. application Ser. No.08/958,260, filed Oct. 27, 1997, now U.S. Pat. No. 5,985,467 which was acontinuation of international application No. PCT/EP96/01565, filed Apr.12, 1996, which designated the United States, and which in turn is acontinuation of U.S. application Ser. No. 08/428,449, filed Apr. 25,1995 and now abandoned, and a continuation of U.S. application Ser. No.08/428,452, filed Apr. 25, 1995 and now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a superalloy component which has appliedthereon a protective coating system with various layers.

2. Description of the Related Art

U.S. Pat. No. 4,055,705 to Stecura et al.; U.S. Pat. No. 4,321,310 toUlion et al.; and U.S. Pat. No. 4,321,311 to Strangman disclose coatingsystems for gas turbine components made from nickel or cobalt-basedsuperalloys. A coating system described comprises a thermal barrierlayer made from ceramic, which in particular has a columnar grainedstructure, placed on a bonding layer or bond coating which in its turnis placed on the substrate and bonds the thermal barrier layer to thesubstrate. The bonding layer is made from an alloy of the MCrAlY type,namely an alloy containing chromium, aluminum and a rare earth metalsuch as yttrium in a base comprising at least one of iron, cobalt andnickel. Further elements can also be present in an MCrAlY alloy;examples are given below. An important feature of the bonding layer is athin layer developed on the MCrAlY alloy and used for anchoring thethermal barrier layer. This layer may be alumina or alumina mixed withchromium oxide, depending on the composition of the MCrAlY alloy and thetemperature of the oxidizing environment where the layer is developed.Eventually, an alumina layer may be placed purposefully by a separatecoating process like PVD.

U.S. Pat. No. 5,238,752 to Duderstadt et al. discloses a coating systemfor a gas turbine component which also incorporates a ceramic thermalbarrier layer and a bonding layer or bond coating bonding the thermalbarrier layer to the substrate. The bonding layer is made from anintermetallic aluminide compound, in particular a nickel aluminide or aplatinum aluminide. The bonding layer also has a thin alumina layerwhich serves to anchor the thermal barrier layer.

U.S. Pat. No. 5,262,245 to Ulion et al. describes a result of an effortto simplify coating systems incorporating thermal barrier layers for gasturbine components by avoiding a bonding layer to be placed below thethermal barrier layer. To this end, a composition for a superalloy isdisclosed which may be used to form a substrate of a gas turbinecomponent and which develops an alumina layer on its outer surfacesunder a suitable treatment. That alumina layer is used to anchor aceramic thermal barrier layer directly on the substrate, eliminating theneed for a special bonding layer to be interposed between the substrateand the thermal barrier layer. In its broadest scope, the superalloyconsists essentially of, as specified in weight percent: 3 to 12 Cr, 3to 10 W, 6 to 12 Ta 4 to 7 Al, 0 to 15 Co, 0 to 3 Mo, 0 to 15 Re, 0 to0.0020 B, 0 to 0.045 C, 0 to 0.8 Hf, 0 to 2 Nb, 0 to 1 V, 0 to 0.01 Zr,0 to 0.07 Ti, 0 to 10 of the noble metals, 0 to 0.1 of the rare earthmetals including Sc and Y, balance Ni.

U.S. Pat. No. 5,087,477 to Giggins, Jr., et al. shows a method forplacing a ceramic thermal barrier layer on a gas turbine component by aphysical vapor deposition process comprising evaporating compoundsforming the thermal barrier layer with an electron beam and establishingan atmosphere having a controlled content of oxygen at the component toreceive the thermal barrier layer.

U.S. Pat. Nos. 5,154,885; 5,268,238; 5,273,712; and 5,401,307, all toCzech et al. disclose advanced coating systems for gas turbinecomponents comprising protective coatings of MCrAlY alloys. The MCrAlYalloys disclosed have carefully balanced compositions to giveexceptionally good resistance to corrosion and oxidation as well as anexceptionally good compatibility to the superalloys used for thesubstrates. The basis of the MCrAlY alloys is formed by nickel and/orcobalt. Additions of further elements, in particular silicon andrhenium, are also discussed. Rhenium in particular is shown to be a veryadvantageous additive. All MCrAlY alloys shown are also very suitable asbonding layers for anchoring thermal barrier layers, particularly in thecontext of the invention which will be described in the following.

The above-mentioned U.S. Pat. No. 5,401,307 to Czech et al. alsocontains a survey over superalloys which are considered useful forforming gas turbine components that are subject to high mechanical andthermal loads during operation. Particularly, four classes ofsuperalloys are given. The respective superalloys consist essentiallyof, as specified in percent by weight:

1. 0.03 to 0.05 C, 18 to 19 Cr, 12 to 15 Co, 3 to 6 Mo, 1 to 1.5 W, 2 to2.5 Al, 3 to 5 Ti, optional minor additions of Ta, Nb, B and/or Zr,balance Ni.

2. 0.1 to 0.15 C, 18 to 22 Cr, 18 to 19 Co, 0 to 2 W, 0 to 4 Mo, 0 to1.5 Ta, 0 to 1 Nb, 1 to 3 Al, 2 to 4 Ti, 0 to 0.75 Hf, optional minoradditions of B and/or Zr, balance Ni.

3. 0.07 to 0.1 C, 12 to 16 Cr, 8 to 10 Co, 1.5 to 2 Mo, 2.5 to 4 W, 1.5to 5 Ta, 0 to 1 Nb, 3 to 4 Al, 3.5 to 5 Ti, 0 to 0.1 Zr, 0 to 1 Hf, anoptional minor addition of B, balance Ni.

4. about 0.25 C, 24 to 30 Cr, 10 to 11 Ni, 7 to 8 W, 0 to 4

5. Ta, 0 to 0.3 Al, 0 to 0.3 Ti, 0 to 0.6 Zr, an optional minor additiveof B, balance cobalt.

Information on modified alumina compounds, in particular aluminacompounds doped with nitrogen, is available from an essay by theinventor entitled "Schichtentwicklungen fur Hochtemperaturanwendungen inthermischen Maschinen" [coating developments for high temperatureapplications in thermic machines] and published under"Fortschritts-Berichte VDI, Reihe 5" [progress reports by VDI, seriesno. 5], ser. No. 345, VDI-Verlag, Dusseldorf, Germany, 1994. That essayalso contains information about processes to deposit such aluminacompounds in the form of layers.

Further information on modified alumina compounds may be derived from anessay by L. Peichl and D. Bettridge entitled "Overlay and DiffusionCoatings for Aero Gas Turbines" and contained in a book entitled"Materials for Advanced Power Engineering, Part One", edited by D.Coutsouradis et al., Kluwer Academic Publishers, Dordrecht, Netherlands,1994, pages 717-740, and from an essay by O. Knotek, E. Lugscheider, F.Loffler and W. Beele, published in: Surface and Coating Techniques, Vol.68/69 (1994), pages 22 to 26.

A standard practice in placing a thermal barrier coating on a substrateof an article of manufacture includes developing an oxide layer on thearticle, either by placing a suitable bonding layer on the article whichdevelops the oxide layer on its surface under oxidizing conditions or byselecting a material for the article which is itself capable ofdeveloping an oxide layer on its surface. That oxide layer is then usedto anchor the thermal barrier layer placed on it subsequently.

Under thermal load, diffusion processes will occur within the article.In particular, diffusion active chemical elements like hafnium,titanium, tungsten and silicon which form constituents of mostsuperalloys used for the articles considered may migrate through theoxide layer and into the thermal barrier layer. The diffusion activechemical elements cause damage to the thermal barrier layer by modifyingand eventually worsening its essential properties. That applies inparticular to a thermal barrier layer made from a zirconia compound likepartly stabilized zirconia, since almost all zirconia compounds mustrely on certain ingredients to define and stabilize their particularproperties. The action of such ingredients is likely to be imparted bychemical elements migrating into a compound, be it by diffusion orotherwise.

To assure that a thermal barrier layer placed on a substrate containingdiffusion active chemical elements keeps its essential properties over atime period as long as may be desired, it is therefore material toprevent migration of diffusion active chemical elements into the thermalbarrier layer.

That aspect has, however, not yet received attention in this art.Heretofore, only oxide layers have been given consideration to anchor athermal barrier layer on a superalloy substrate regardless of theirtransmission of diffusing chemical elements to the thermal barrierlayer.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a superalloycomponent with a protective coating system, which overcomes theabove-mentioned disadvantages of the heretofore-known devices of thisgeneral type and which keeps to a minimum or prevents the transmissionof diffusing elements through an anchoring layer to a thermal barrierlayer by modifying the anchoring layer suitably.

With the foregoing and other objects in view there is provided, inaccordance with the invention, an article of manufacture, comprising: asubstrate formed of a nickel or cobalt-based superalloy; an anchoringlayer disposed on the substrate, the anchoring layer comprising an oxidecompound doped with nitrogen; and a ceramic coating disposed on theanchoring layer.

In accordance with a preferred feature of the invention, the oxidecompound comprises alumina and/or chromium oxide. Particularly, theoxide compound consists essentially of alumina.

In accordance with an added feature of the invention, the articleincludes a diffusion active chemical element covered by the anchoringlayer. The diffusion active chemical element is preferably an elementselected from the group consisting of hafnium, titanium, tungsten andsilicon. In particular, the diffusion active element is contained in thesubstrate or a bonding layer disposed thereon.

In accordance with another feature of the invention, the ceramic coatingincludes ZrO₂. In a further development, the ceramic coating consistsessentially of ZrO₂ and a stabilizer selected from the group consistingof Y₂ O₃, CeO₂, LaO and MgO.

In a preferable embodiment, the anchoring layer has a thickness of lessthan 3 μm.

In accordance with a further feature of the invention, the anchoringlayer contains from 1 to 10 atom percent of nitrogen, and preferablyfrom 2 to 5 atom percent.

In accordance with again an added feature of the invention, theanchoring layer is also doped with a further chemical element, thefurther chemical element being also covered by the anchoring layer. Inone embodiment of the invention, the further chemical element ischromium. In particular, the further chemical element is contained inthe substrate or a bonding layer disposed thereon.

In accordance with again an additional feature of the invention, theanchoring layer is doped with a further chemical element, and thefurther chemical element is also present in the ceramic coating.

In accordance with again a further feature of the invention, the articleis provided with a bonding layer interposed between the substrate andthe anchoring layer.

In preferred embodiments, the bonding layer is formed of a metalaluminide, or it is formed of an MCrAlY alloy.

In accordance with yet another feature of the invention, the anchoringlayer is doped with a further chemical element, and the further elementis also present in the bonding layer.

In accordance with yet an added feature of the invention, the substrate,the bonding layer (if present), the anchoring layer and the ceramiccoating form a gas turbine component. In particular, the gas turbinecomponent is a gas turbine airfoil component comprising a mountingportion and an airfoil portion, the mounting portion being adapted tofixedly hold the component in operation and the airfoil portion beingadapted to be exposed to a gas stream streaming along the component inoperation, the anchoring layer and the ceramic layer being disposed onthe airfoil portion.

With the above-mentioned and other objects in view, there is alsoprovided, in accordance with the invention, a method of applying aceramic coating to an article of manufacture having a substrate formedof a nickel or cobalt-based superalloy. Particularly, the substrate mayhave a bonding layer placed thereon, as described above. The methodcomprises the following steps:

placing an anchoring layer comprising an oxide compound doped withnitrogen on a substrate formed of a nickel or cobalt-based superalloy;and

placing a ceramic coating on the anchoring layer.

In accordance with an additional mode of the invention, the step ofplacing the anchoring layer is performed by a physical vapor depositionprocess. Preferably, a physical vapor deposition process includingsputtering or electron beam evaporation is used.

In accordance with another mode of the invention, the step of placingthe anchoring layer comprises:

placing a layer comprising an other oxide compound essentially free ofnitrogen on the substrate;

establishing an atmosphere containing nitrogen around the layer; and

creating the anchoring layer by subjecting the layer and the atmosphereto an elevated temperature and diffusing the nitrogen into the layer.

The ceramic coating may also be placed onto the system by physical vapordeposition (PVD) process.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is illustrated and described herein as embodiedin a superalloy component with a protective coating system, it isnevertheless not intended to be limited to the details shown, sincevarious modifications and structural changes may be made therein withoutdeparting from the spirit of the invention and within the scope andrange of equivalents of the claims.

The construction of the invention, however, together with additionalobjects and advantages thereof will be best understood from thefollowing description of the specific embodiments when read inconnection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary cross-sectional view of a substrate having aprotective coating system incorporating a ceramic coating placedthereon; and

FIG. 2 is a perspective view of a gas turbine airfoil componentcomprising the substrate and protective coating system shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the figures of the drawing in detail and first,particularly, to FIG. 1 thereof, there is seen a substrate 1 of anarticle of manufacture, in particular a gas turbine component, which inoperation is subject to heavy thermal load and concurrently to corrosiveand erosive attack. The substrate 1 is formed of a material which issuitable to provide strength and structural stability when subjected toa heavy thermal load and eventually an additional mechanical load bysevere forces like centrifugal forces. A material which is widelyrecognized and employed for such a purpose in a gas turbine engine is anickel or cobalt-based superalloy.

In order to limit the thermal load imposed on the substrate 1, a thermalbarrier layer 2 is placed thereon. The thermal barrier layer 2 is madefrom a columnar grained ceramic, in particular consisting essentially ofa stabilized or partly stabilized zirconia, as explained above. Thethermal barrier layer 2 is anchored to the substrate 1 by means of anintermediate layer 3. The intermediate layer 3 is made by placing abonding layer 4 on the substrate 1, which bonding layer 4 consists of anMCrAlY alloy and preferably of an MCrAlY alloy as disclosed in one ofU.S. Pat. Nos. 5,154,885; 5,268,238; 5,273,712; and 5,401,307, anddeveloping an anchoring layer 5 on the bonding layer 4, as explainedsubsequently. The bonding layer 4 has certain functions in common with abonding layer as known from the state of the art and in particular has atight bond to the substrate 1. The anchoring layer 5 serves as an anchorfor the thermal barrier layer 2. Both the bonding layer 4 and theanchoring layer 5 form the intermediate layer 3.

The drawing is not intended to show the thicknesses of the layers 4 and5 to scale; the thickness of the anchoring layer 5 might in reality bevery much less than the thickness of the bonding layer 4 and amount onlyto a few atomic layers, as specified hereinabove.

The anchoring layer 5 essentially consists of an oxide compound, namelyalumina, doped with nitrogen. The nitrogen content need not be veryhigh, and a nitrogen content of a few atom percent is considered to beeffective.

The effect of the nitrogen doping in the anchoring layer 5 relies on thefact that the nitrogen atoms distributed in the oxide crystals, whichare ion crystals made from positively charged metal ions and O²⁻ ions,introduce imbalances into the distribution of electric charges in theion crystal and thus hinder diffusion of atoms through the ion crystal.It can be said that an addition of a minor quantity of atoms effectivelydifferent from the atoms or related ions making up the crystalintroduces irregularities into the crystal and renders it opaque orimpenetrable for diffusing atoms, which no longer experience a regularpattern of constituents essential for easy penetration through acrystal. An addition of nitrogen in a considerably large amount and farbeyond any addition which might be considered to be a doping could ofcourse give rise to a complete restructuring of an oxide crystal into acrystal consisting of a regular pattern of metal, oxygen and nitrogenconstituents.

The anchoring layer 5 can be made by several methods, in particular by aphysical vapor deposition process like electron beam PVD, sputter ionplating and cathodic arc-PVD, or by thermal treatment of an oxidecompound layer in a nitrogen-containing atmosphere. Such thermaltreatment is in particular carried out at a temperature within a rangebetween 700° C. and 1100° C. A nitrogen-containing atmosphere may alsoserve to provide the nitrogen for a PVD-process, which comprisesevaporating the required metal and oxygen compound from a suitablesource and adding the nitrogen from the atmosphere. Particularly, thenitrogen-containing atmosphere essentially consists of argon, oxygen andnitrogen, at a total pressure between 10⁻² Pa and 1 Pa.

FIG. 2 shows the complete gas turbine component, namely a gas turbineairfoil component 6, in particular a turbine blade. The component 6 hasan airfoil portion 7, which in operation forms an "active part" of thegas turbine engine, a mounting portion 8, at which the component 6 isfixedly held in its place, and a sealing portion 9, which forms a sealtogether with adjacent sealing portions of neighboring components toprevent an escape of a gas stream 10 flowing along the airfoil portion 7during operation.

The section of FIG. 1 is taken along the line I--I in FIG. 2.

Referring again to FIG. 1, particular advantages of the novelcombination of the anchoring layer 5 and the thermal barrier layer 2 canbe summarized as follows: As the anchoring layer 5 has a high content ofcompounds made up of metal and oxygen, whereof an exact chemical formulacan hardly been given due to the distorting action of the nitrogen alsopresent, it is indeed very suitable for anchoring a thermal barrierlayer 2. That thermal barrier layer 2 may expediently be deposited onthe substrate 1 immediately after deposition of the anchoring layer 5and in particular within the same apparatus and by using as much aspossible installations which have been already in use for depositing theanchoring layer 5. The combination of the anchoring layer 5 and thethermal barrier layer 2 thus made has all the advantages of suchcombinations known from the prior art and additionally features asubstantially prolonged lifetime due to the suppression of migration ofdiffusion active elements into the thermal barrier layer 2.

I claim:
 1. A method of coating an article of manufacture having asubstrate formed of a nickel or cobalt-based superalloy, the methodwhich comprises:forming an intermediate layer on the substrate of thenickel or cobalt-based superalloy, the intermediate layer comprising ananchoring layer formed of an oxide compound doped with nitrogen; andplacing a ceramic coating on the intermediate layer.
 2. The methodaccording to claim 1, wherein the forming step comprises interposing abonding layer between the substrate and the anchoring layer.
 3. Themethod according to claim 1, wherein the step of forming the anchoringlayer is performed by a physical vapor deposition process.
 4. The methodaccording to claim 1, wherein the step of forming the anchoring layercomprises:placing a layer comprising an oxide compound which issubstantially free of nitrogen on the substrate; establishing anatmosphere containing nitrogen around the layer; and subjecting thelayer and the atmosphere to elevated temperature and diffusing thenitrogen into the layer.
 5. The method according to claim 1, wherein thestep of forming the anchoring layer comprises:placing a layer comprisingan oxide compound which is substantially free of nitrogen onto thebonding layer; establishing an atmosphere containing nitrogen around thelayer; and subjecting the layer and the atmosphere to elevatedtemperature and diffusing the nitrogen into the layer.
 6. The methodaccording to claim 1, wherein the step of placing the ceramic coatingcomprises depositing ceramic on the anchoring layer by physical vapordeposition.